You can travel light, or you can pack every piece of gear available to you to cover every conceivable shooting scenario. What do you want to do ?
- On a family excursion to NH last weekend, main goal was documentary memories - I brought only the gripped 5DII and 28-300L, in a Lowepro Toploader Pro 75 AW.
- Visiting family in CA for a few days a couple of months ago, goals were documentary memories including indoor portraits and ambient shooting - brought gripped 5DII, 28-300L, 35L, 135L, and 430EX II in a Lowepro Flipside 300 packed in a Storm im2500 hard case.
- On a 3-week adoption trip to China last year, goal was documentary memories and a photographic chronicle of the trip for our then-new daughter. Even though we were traveling with our older daughter, I brought the gripped 5DII, 16-35L II, 24-105L, 70-200/2.8L IS II, TS-E 24L II, 35L, 1.4x II TC, and 430EX II, all in a Lowepro Flipside 400 AW packed in the Storm im2500.
Some key points - use a bag that's the right size for the gear you're bringing; a backpack style bag (vs. sling) supports the weight better. When carrying lots of gear (like the China trip) I also pack a smaller bag in with the main luggage (Lowepro Toploader 65 AW and Lens Case 1W, on that trip) to carry body + 1-2 lenses, and leave the rest in the hotel, locked in the hard case (I use TamperSeal SearchAlert locks).
First off, just bring one flash, probably the 270. Second, I agree (and repeat) that the 70-300 non-L is not worth taking - you'd be best off with the L version, and still better off with 200mm and cropping. You could also consider the 200mm f/2.8L II prime. The 70-300 DO is another option as suggested, but don't buy new - way overpriced for what it is, I bought one for $750 (and later, sold it for the same price). The IQ is not stellar, but ok (needs a contrast boost and sharpening in post), it's better that the 70-300 non-L, but the L is much better; the DO has bad zoom creep, too.
For SeaWorld, the SD Zoo, and especially the Safari Park (was called Wild Animal Park when I lived in SD), you'll want 300mm, IMO.